Field of the Invention
The present invention relates to the technology of exposing a train of droplet targets to laser radiation. It refers to a method for controlling an interaction between droplet targets and a high power and high-repetition-rate laser beam.
Description of Related Art
The generation of spatially stable micro-sized droplets has applications in several scientific and industrial fields. These include nuclear physics, where droplet are used as fuel for inertial fusion confinement (ICF) or semiconductor industry, where micro-sized droplets are used as fuel for Soft X rays Laser Produced Plasma (LPP) sources. In particular, the semiconductor industry requires the fabrication of microchips with node sizes below 24 nm. For this purpose next generation photolithography techniques are required, capable of providing radiation emission in the Extreme Ultraviolet (EUV) wavelength region. Current bright EUV radiation sources emit at 13.5 nm in the 2% bandwidth (called in-band radiation). The requirement of 13.5 nm in-band radiation is due the available multilayer mirrors. These mirrors are made from 40-60 alternating layers of molybdenum and silicon and reflect about the 70% of incoming EUV radiation at 13.5 nm. In order to develop an EUV light source, the scientific and industrial communities have devoted their attention to elements such as xenon and tin. If xenon or tin is brought in a state of plasma, the atomic transitions identified in the plasma are the desired ones in order to obtain photon emission in the EUV wavelength region.
Plasma EUV sources are mainly developed using Discharge Produced Plasmas (DPP) when xenon is used as fuel, or Laser Produced Plasma (LPP) when tin is used as the fuel. For the LPP, a laser beam (generally a ND:YAG or a CO2 laser) having a power on the order of kilowatts with pulse lengths on the order of nanoseconds is focused onto solid or liquid tin targets. The liquid tin is generally in the form of micro-sized droplet targets. A laser power density of about 1010-1011 W/cm2 will be delivered to the target, thus generating first the vaporization of the solid material and then the plasma. At these power densities the ions within the plasma have high enough charge states so that emission occurs in the EUV region, in particular at 13.5 nm. The emitted in-band radiation is then reflected by the collector mirror and directed towards the so-called intermediate focus (IF) point. In a typical scanner, a number of illumination optics directs the EUV light from the IF to the reticle (or mask), before it is projected onto the wafer.
The principal requirements for the LPP source include the use of mass limited liquid tin droplet targets. The advantage of using droplet targets is that they can ensure long-term operation of the source as they form a regenerative target material, capable of operating at high repetition rates. At the same time, the use of mass limited targets reduces the emission of high-energy plasma particles (such as ions or neutrals) that damage the EUV radiation collection optics. On the other hand, many physical and chemical processes have to be taken into account when mass limited droplet target are generated. During source operation, the droplet target can be affected by spatial and temporal instabilities. Mainly two types of instabilities are distinguished: lateral instabilities, i.e. spatial position drifts perpendicular to the droplet train, and axial instabilities, i.e. spatial position drifts along the droplet train. Lateral instabilities imply variations of the deposited laser energy on the droplet target, as the droplet target position drifts in the plane of the incoming laser. Axial instabilities imply variations of the droplet spacing and lead to temporal jitters, as the droplets do not pass the laser focus at a constant frequency. This leads to droplet/laser pulse de-synchronization.
As a consequence, both types of instabilities induce variations in the EUV emission. Therefore, a key requirement for an EUV source is to deliver droplets to the irradiation site with maximum stability. Since the causes of these instabilities are not always well known, it is fundamental to develop high precision control systems capable of maintaining the droplet target position at the high power laser focus position with a specific frequency. The controller needs to compensate for spatial and temporal instabilities. Control systems can have a closed loop or and open loop. An open loop control system simply consists of the synchronization of the electrical signals controlling the droplet target and the laser pulse. A closed loop control system is able to return the droplet to the main laser focus position if a lateral instability occurs. It is clear that a closed loop control system is preferable for the correct operation of the EUV source. The position of the droplet target needs to be adjusted in a micrometer scale range and frequently in time. That means that the spatial resolution and response time of the controller needs to be as high as possible. Different system can be built for controlling the target position, employing different types of sensors. Previous control systems were developed with the use of CCD sensors, photodiode sensors etc. However, previous system employed a combination of numerous types of sensors, in order to achieve high precision for the controller in both spatial and temporal domain.